Cresylic acid is an important commercial product widely used in the manufacture of chemical, agrichemical, pharmaceutical, and industrial intermediate products. Unfortunately, very few cresylic acid members are commercially synthesized. For example, the lowest molecular weight member of the cresylic acid family, phenol, is produced synthetically in very large quantities. Similarly, the methylphenols (known as cresols) are also produced synthetically, but in much smaller quantities. On the other hand, the dimethylphenols (known as xylenols) and other alkylated phenols are not commercially synthesized to any appreciable extent with the exception of 2,6-xylenol. As such, the majority of cresylic acid isomers used in industry today are recovered from natural sources, such as partially refined petroleum and coal via coking, gasification, and liquefaction
The cresylic acid recovered from these sources, however, is heavily contaminated with aromatic organic compounds including hydrocarbons containing hetero-atoms such as nitrogen, sulfur, and oxygen. These impurities must be removed in order to make marketable products. Methoxy-substituted phenols, such as guaiacol and methyl guaiacol, comprise a particularly troublesome group of contaminants Since guaiacol, an ortho-methoxy phenol, boils near the boiling points of meta-cresol and para-cresol, and methyl guaiacol, a methoxy cresol, boils in the range of xylenols, they cannot be separated from the cresylic acid fractions by conventional distillation. The presence of such methoxyaromatic impurities significantly reduces the commercial value of cresylic acid as a raw material for high quality plastics and resins. To be useful, the various isomers of cresylic acid must be separated from these impurities and often from each other, and therein lies the problem because, heretofore, there has been no simple process for physically or chemically separating guaiacols from cresylic acid. In the past, the guaiacol was destroyed in the presence of the cresylic acid but with a considerable decrease in cresylic acid yield. Moreover, such destruction had been accomplished only with much difficulty and with the resultant loss of cresylic acid yield to byproducts, most of them unwanted heavies and coke.
Considerable academic research has been reported relating to the removal of methoxy compounds and to the demethylation of phenols. This work is reported in articles, such as J. Lawson and M. Klein, "Influence of Water on Guaiacol Pyrolysis," Ind. Eng. Chem. Fundam., 24:203, 1985; R. Ceylan and J. Bredenberg, "Hydrogenolysis and Hydrocracking of the Carbon-Oxygen Bond. 2. Thermal Cleavage of the Carbon-Oxygen Bond in Guaiacol," Fuel, 61:377, 1982; and A. Vuori and J. Bredenberg, "Hydrogenolysis and Hydrocracking of the Carbon-Oxygen Bond. 4. Thermal and Catalytic Hydrogenolysis of 4-Propylguaiacol," Holzforschung, 38:133, 1984.
Likewise, a dealkylation process involving the reaction of acetic acid in the presence of an alumina-silica catalyst in the liquid phase is described in U.S. Pat. No. 2,697,732. The rearrangement of alkyl phenyl ethers to ortho-alkyl phenols by heating at temperatures from 75.degree. C. to 200.degree. C. in the presence of alumina is described in U.S. Pat. Nos. 4,447,657 and 4,381,413. A process for the hydrolysis of alkyl-aryl ethers in the presence of a carboxylate, preferably an alkali metal carboxylate, catalyst is described in U.S. Pat. No. 4,473,713. Recent processes have described the removal of guaiacols by pyrolysis (U.S. Pat. No. 5,146,010) and U.S. Pat. No. 5,177,269 describes a process for the removal of guaiacol from naturally occurring cresylic acid feed by heating the feed with a strong acid, particularly hydrochloric acid or sulfuric acid, to produce a guaiacol free product.
Notwithstanding the considerable amount of investigation of this problem of guaiacol removal, the search for totally satisfactory results continues. It is an object of this invention to provide a process for the destruction of guaiacols in the presence of cresylic acids so they may be used for other purposes.